1. Field of the Invention
The invention relates to a process for the continuous hydrolytic polymerization of laurolactam, in which hydrolytic cleavage is first used to prepare a prepolymer which is subsequently condensed to give a high-molecular-weight polyamide 12 product.
2. Discussion of the Background
The conventional method of preparing polyamide 12 from laurolactam involves a batchwise hydrolytic polymerization in a stirred reaction vessel, as is described in various patent applications (see, for example, U.S. Pat. Nos. 3,410832; 3,799,899 and 4,837,297). Unfortunately, this method possesses, inter alia, the following inherent disadvantages. First, on the one hand, a high water content is desirable, since water has strongly accelerating effect on the ring cleavage and thus on the prepolymerization. On the other hand, however, this acceleration gives rise to high steam pressures at the temperatures required for the reaction. Since the costs of large, stirred reaction vessels increase greatly with the permissible operating pressure, only relatively small amounts of water can be used, resulting in long reaction times. This leads to high manufacturing costs.
Second, the batchwise procedure results in only sequential utilization of individual equipment items, such as conveying facilities, discharge filters or granulators. These equipment items therefore have to be designed for throughputs which are much higher than the average throughput of the total plant, which leads to high costs.
Third, in the batchwise operation of pressure vessel plants, problems usually result from material which remains in the vessel when the latter is discharged. Prolonged residence times result in interfering secondary reactions of the polyamide 12 formed. Product residues in the reactor and in the lines can, for example, lead to gel-like impurities in the next batch.
The disadvantages of a batchwise mode of operation can be avoided by a continuous polymerization process.
For the polymerization of caprolactam to give polyamide 6, a continuous procedure has been the state of the art for some time, as described in, for example, H. Ludewig, Faserfor-schung Textiltechn. 2, 341-355 (1951). However, the processes described for caprolactam are, without exception, not applicable to laurolactam, since this material behaves completely differently from caprolactam in respect of the polymerization conditions required (water content, pressure, temperature, residence time).
U.S. Pat. No. 4,539,391 describes a process for the continuous polymerization of caprolactam, in which the lactam containing from 1 to 25% by weight of water is heated to a temperature of from 220.degree. to 280.degree. C. in a prepolymerization zone at a pressure of from 1 to 10 bar with simultaneous evaporation of the water over a residence time of from 1 to 10 minutes and is subsequently further polymerized in a polymerization zone with continuous removal of the steam. However, use of laurolactam in place of the lactams having from 7 to 12 ring members described in this document gives virtually no conversion.
There have hitherto been few developments specifically for the continuous hydrolytic polymerization of laurolactam. All the inventions published in this field have considerable disadvantages which greatly limit the economics or the product properties.
SU-A 12 08 044 necessitates the use of phosphoric acid as catalyst; the laurolactam conversion reaches only 99%. The remaining residual monomer content of 1% causes problems in processing and use of the product. Commercial use of these products would require a preceding, complicated removal of monomer. In addition, the use of such a strongly acid catalyst would have the disadvantage that the polylaurolactam thus prepared experiences increased hydrolytic degradation in its processing or in use at elevated temperature; in addition, the polymerization reactors and the processing machines are subjected to increased corrosion.
JP-A 60 041 647 covers only the region of very high temperatures and pressures as process parameters. However, under these conditions there is formation of gel particles and the color is impaired. Furthermore, only oligomers can be obtained at first, and the further polycondensation of these requires, e.g. as described in JP-A 61 166 833, complicated techniques and equipment items such as degassing screw machines.
The process described in JP-A 49 021 313 leads to reaction times which give no substantial advantages in comparison with non-continuous batch procedures (residence time a total of 13 hours). The procedure described leads to gelling of the product in the second reactor.
In U.S. Pat. No. 4,077,946, phosphoric acid has, according to the invention, to be present as catalyst; the depressurization process is carried out isothermally at great expense; furthermore, it has been found that the monomer removal described cannot be carried out in practice. U.S. Pat. No. 4,077,946 can be evaluated similarly.
U.S. Pat. No. 5,283,315 describes a continuous polymerization process in a complicated multi-component reactor which has to be kept under a temperature gradient. To prepare the finished polymer, a condensation facility with stirrers is required. The water contents given in the polymerization (from 1 to 10%) lie outside the values for a maximum space-time yield. The residence times required as a result for the prepolymerization of from 7 to 8 hours offer no advantage in comparison with the batchwise process.